For naming conventions we follow the Taligent rules. They have written a very large body of C++ and their rules seem well thought out. No need to invent something new. The only addition/change we made is to append an _t to typedefs and simple structs, e.g.:

typedefint Int_t ;struct Simple_t { ..... };

Addherence to the rules is mandatory. After a while one really gets used to the fact that all class fields start with an f followed by a capitalized word, fEnergy, or that TStreamerInfo is a class. If the convention is sporadically violated debugging becomes a nightmare. The usage of a standard begin letter or token for the different types also makes it easy to parse and search the code using simple tools.

Also here the Taligent guide is quite reasonable. Of course, no class data member should ever be public. Make the data fields always private. Or protected, if you want to grant an inherited class direct access.

Add trivial get or setters directly in the class definition. This improves reading time since one does not have to look for it somewhere else. Add more complex inlines (longer than one line) at the bottom of the .h file. Creating separate .icc files increases the build time, the complexity of the build system and, more importantly, increases the number of files one possibly has to scan to find a piece of code.

In the class definition we first declare all private data members, followed by the private static members, the private methods and the private static methods. Then the protected members and methods and finally the public methods (no public data members). We put private members first since that is the language default and it gives the developer a quick view on what data members are used in a class.

Avoid the use of raw C types like int, long, float, double when using data that might be written to disk. For example, the sizes of int and long are machine dependent. On 32 bit machines ints and longs are 32 bits, but on 64 bit processors an int can be either 32 or 64 bits and a long 64 bits, depending on the processor. For portability reasons and consistent numerical results use the typedefs provided by ROOT's Rtypes.h for the basic raw C types. E.g.: Double_t, Float_t, Int_t etc.

In ROOT 5 all classes are in the ROOT namespace. Some packages will be in a sub-namespace, e.g. ROOT::Reflex. For backward compatibility with the previous versions of ROOT, where all classes were in the global namespace, we have by default using namespace ROOT; in all headers. However, this can be turned off by defining the USE_ROOT_NAMESPACE macro.

Note the explicit checks to avoid unnecessarily opening already included header files. For large systems this kind of defensive measures can make quite a difference in compile time. Also never include a header file when a forward declaration is enough. On include header files for base classes or classes that are used by value in the class definition.

Here we describe our preferred coding style. Coding style is very personal and we don't want to force our views on anybody. But for any contributions to the ROOT system that we have to maintain we would like you to follow our coding style.

To be able to keep as much code as possible in the visible part of the editor of to avoid over abundant line wrapping we use indentation of 3 spaces. No tabs since they give the code always a different look depending on the tab settings of the original coder. If everything looks nicely lined up with a tab setting of 4 spaces, it does not look so nicely anymore when the tab setting is changed to 3, 5, etc. spaces.

The other issue that always comes up in C/C++ styling is the placement of braces and spaces. Unlike the indent size, there are few technical reasons to choose one placement strategy over the other, but the preferred way, as shown to us by the prophets Kernighan and Ritchie, is to put the opening brace last on the line, and put the closing brace first, thus:

if(x is true){
we do y
}

However, there is one special case, namely functions: they have the opening brace at the beginning of the next line, thus:

int function (int x){
body of function
}

Functions are special (you can't nest them in C/C++).
Note that the closing brace is empty on a line of its own, except in the cases where it is followed by a continuation of the same statement, ie a while in a do-statement or an else in an if-statement, like this:

do{
body of do- loop
}while( condition );

and

if(x == y){
...
}elseif(x > y){
...
}else{
...
}

Note that this brace-placement also minimizes the number of empty (or almost empty) lines, without any loss of readability. Thus, as the supply of new-lines on your screen is not a renewable resource (think 25-line terminal screens here), you have more empty lines to put comments on.

Notice also in the above examples the usage of spaces around keywords, operators and parenthesis/braces. Avoid the following free styles:

If you don't have access to ClangFormat, astyle can be useful. Starting from a code like this:

int aap (int inp ){if( inp >0){return0;int a =1;if( inp ==0&& a ==1){printf(>"this is a very long line that is not yet ending" , a, inp, a, inp, a , inp );
a += inp ;return a ;}}else{return1;}if( inp ==0)return-1;return1;}

You will find back like this:

int aap (int inp){if(inp >0){return0;int a =1;if(inp ==0&& a ==1){printf("this is a very long line that is not yet ending" , a, inp, a, inp, a, inp );
a += inp ;return a ;}}else{return1;}if(inp ==0)return-1;return1;}